[PATCH] tracing: Fix warning in trace_buffered_event_disable()

[Zheng Yejian]

Warning happened in trace_buffered_event_disable() at
  WARN_ON_ONCE(!trace_buffered_event_ref)

  Call Trace:
   ? __warn+0xa5/0x1b0
   ? trace_buffered_event_disable+0x189/0x1b0
   __ftrace_event_enable_disable+0x19e/0x3e0
   free_probe_data+0x3b/0xa0
   unregister_ftrace_function_probe_func+0x6b8/0x800
   event_enable_func+0x2f0/0x3d0
   ftrace_process_regex.isra.0+0x12d/0x1b0
   ftrace_filter_write+0xe6/0x140
   vfs_write+0x1c9/0x6f0
   [...]

The cause of the warning is in __ftrace_event_enable_disable(),
trace_buffered_event_enable() was called once while
trace_buffered_event_disable() was called twice.
Reproduction script show as below, for analysis, see the comments:
 ```
 #!/bin/bash

 cd /sys/kernel/tracing/

 # 1. Register a 'disable_event' command, then:
 #    1) SOFT_DISABLED_BIT was set;
 #    2) trace_buffered_event_enable() was called first time;
 echo 'cmdline_proc_show:disable_event:initcall:initcall_finish' > \
     set_ftrace_filter

 # 2. Enable the event registered, then:
 #    1) SOFT_DISABLED_BIT was cleared;
 #    2) trace_buffered_event_disable() was called first time;
 echo 1 > events/initcall/initcall_finish/enable

 # 3. Try to call into cmdline_proc_show(), then SOFT_DISABLED_BIT was
 #    set again!!!
 cat /proc/cmdline

 # 4. Unregister the 'disable_event' command, then:
 #    1) SOFT_DISABLED_BIT was cleared again;
 #    2) trace_buffered_event_disable() was called second time!!!
 echo '!cmdline_proc_show:disable_event:initcall:initcall_finish' > \
     set_ftrace_filter
 ```

To fix it, IIUC, we can change to call trace_buffered_event_enable() at
fist time soft-mode enabled, and call trace_buffered_event_disable() at
last time soft-mode disabled.

Fixes: 0fc1b09ff1ff ("tracing: Use temp buffer when filtering events")
Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
---
 kernel/trace/trace_events.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 5d6ae4eae510..578f1f7d49a6 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -611,7 +611,6 @@ static int __ftrace_event_enable_disable(struct trace_event_file *file,
 {
 	struct trace_event_call *call = file->event_call;
 	struct trace_array *tr = file->tr;
-	unsigned long file_flags = file->flags;
 	int ret = 0;
 	int disable;
 
@@ -635,6 +634,8 @@ static int __ftrace_event_enable_disable(struct trace_event_file *file,
 				break;
 			disable = file->flags & EVENT_FILE_FL_SOFT_DISABLED;
 			clear_bit(EVENT_FILE_FL_SOFT_MODE_BIT, &file->flags);
+			/* Disable use of trace_buffered_event */
+			trace_buffered_event_disable();
 		} else
 			disable = !(file->flags & EVENT_FILE_FL_SOFT_MODE);
 
@@ -673,6 +674,8 @@ static int __ftrace_event_enable_disable(struct trace_event_file *file,
 			if (atomic_inc_return(&file->sm_ref) > 1)
 				break;
 			set_bit(EVENT_FILE_FL_SOFT_MODE_BIT, &file->flags);
+			/* Enable use of trace_buffered_event */
+			trace_buffered_event_enable();
 		}
 
 		if (!(file->flags & EVENT_FILE_FL_ENABLED)) {
@@ -712,15 +715,6 @@ static int __ftrace_event_enable_disable(struct trace_event_file *file,
 		break;
 	}
 
-	/* Enable or disable use of trace_buffered_event */
-	if ((file_flags & EVENT_FILE_FL_SOFT_DISABLED) !=
-	    (file->flags & EVENT_FILE_FL_SOFT_DISABLED)) {
-		if (file->flags & EVENT_FILE_FL_SOFT_DISABLED)
-			trace_buffered_event_enable();
-		else
-			trace_buffered_event_disable();
-	}
-
 	return ret;
 }
 
-- 
2.25.1